We have developed an antibody fusion protein composed of avidin fused to a human IgG3 specific for the transferrin receptor (TfR). Our initial goal was to use this molecule (anti-TfR IgG3-Av) as a universal vector to deliver biotinylated agents into cancer cells. We have found that anti-TfR IgG3-Av effectively delivers biotinylated molecules into cancer cells by receptor mediated endocytosis and that these molecules remain active after their internalization. Furthermore, we have unexpectedly discovered that anti-TfR IgG3- Av, but not a recombinant anti-TfR IgG3 or a non-specific IgG3-Av, possesses a strong intrinsic anti- proliferative/pro-apoptotic activity against hematopoietic malignant cell lines. Importantly, this cytotoxic activity may be further enhanced by the addition of biotinylated compounds. We now hypothesize that anti- TfR IgG3-Av can be used alone or in combination with other agents as a novel drug against an incurable plasma cell malignancy: multiple myeloma (MM). We propose to determine the effect of anti-TfR IgG3-Av on both proliferation and apoptosis of selected human MM cell lines and to define the mechanism responsible for the inhibition of proliferation and induction of apoptosis. We will also examine the additive/synergistic effect of combining anti-TfR IgG3-Av with other cytotoxic/sensitizing agents, such as biotinylated Pseudomonas exotoxin A (PE) and drugs currently used in the treatment of MM such as Thalidomide and Dexamethasone. Based on our findings we will test the efficacy of anti-TfR IgG3-Av alone or combined with other anti-cancer drugs against primary myeloma cells obtained from MM patients and against human MM tumors growing in immunodeficient mice. Thus, the proposed experiments will result in a better understanding of the mechanism of action of anti-TfR IgG3-Av and will indicate if this novel therapeutic has potential for use in the treatment of MM. We anticipate that the utility of this therapeutic will not be restricted to the elimination of myeloma cells in vivo but can also be used for in vitro approaches including the efficient purging of myeloma cells during ex vivo expansion of hematopoietic progenitor-cells for use in autologous transplantation in MM patients. We would like to stress that the impact of the results obtained from the present studies is not restricted to MM. Similar approaches can be applied to other hematopoietic malignancies such as leukemias and lymphomas.